Signal isolators using micro-transformers

ABSTRACT

A logic signal isolator comprising a transformer having a primary winding and a secondary winding; a transmitter circuit which drives said primary winding in response to a received logic signal, such that in response to a first type of edge in the logic signal, a signal of a first predetermined type is supplied to the primary winding and in response to a second type of edge in the logic signal, a signal of a second predetermined type is supplied to said primary winding, the primary winding and the transmitter being referenced to a first ground; and the secondary winding being referenced to a second ground which is galvanically isolated from the first ground and said secondary winding supplying to a receiver circuit signals received in correspondence to the signals provided to the primary winding, the receiver reconstructing the received logic signal from the received signals.

RELATED APPLICATIONS

This application is a Continuation application of U.S. patentapplication Ser. No. 11/481,997, filed on Jul. 6, 2006, which is aContinuation application of U.S. patent application Ser. No. 10/834,959filed on Apr. 29, 2004, U.S. Pat. No. 7,075,329, which claims benefit ofand priority to U.S. Provisional Application Ser. No. 60/466,602entitled “BI-DIRECTIONAL SIGNAL ISOLATORS USING MICRO-TRANSFORMERS,”filed on Apr. 30, 2003. All of the above applications are herebyincorporated by reference herein.

FIELD OF THE INVENTION

The subject invention relates to signal isolators, more particularlydigital signal isolators, and even more particularly to digital signalisolators employing transformers to establish an isolation barrier.

BACKGROUND OF THE INVENTION

In a variety of environments, signals must be transmitted betweendiverse sources and circuitry using those signals, while maintainingelectrical (i.e., galvanic) isolation between the sources and the usingcircuitry. Isolation may be needed, for example, betweenmicrocontrollers, on the one hand, and devices or transducers which usemicrocontroller output signals, on the other hand. Electrical isolationis intended, inter alia, to prevent extraneous transient signals,including common-mode transients, from inadvertently being processed asstatus or control information, or to protect the equipment from shockhazards or to permit the equipment on each side of an isolation barrierto be operated at a different supply voltage, among other knownobjectives and uses.

A variety of isolation techniques are known, including the use ofoptical isolators that convert input electrical signals to light levelsor pulses generated by light emitting diodes, and then to receive andconvert the light signals back into electrical signals. Isolators alsoexist which are based upon the use of Hall effect devices,magneto-resistive sensors, capacitive isolators and coil- ortransformer-based isolators.

Optical isolators, which are probably the most prevalent, presentcertain well-known limitations. Among other limitations, they requiresignificant space on a card or circuit board, they draw a large current,they do not operate well at high frequencies, and they are veryinefficient. They also provide somewhat limited levels of isolation. Toachieve greater isolation, optical-electronic isolators have been madewith some attempts at providing an electrostatic shield between theoptical transmitter and the optical receiver. However, a conductiveshield which provides a significant degree of isolation is notsufficiently transparent for use in this application.

In the area of non-optical isolation amplifiers for use in digitalsignaling environments, U.S. Pat. No. 4,748,419 to Somerville, shows thedifferentiation of an input data signal to create a pair of differentialsignals that are each transmitted across high-voltage capacitors tocreate differentiated spike signals for the differential input pair.Circuitry on the other side of the capacitor barrier has a differentialamplifier, a pair of converters for comparing the amplified signalagainst high and low thresholds, and a set/reset flip-flop to restorethe spikes created by the capacitors into a logic signal. In such acapacitively-coupled device, however, during a common mode transientevent, the capacitors couple high, common-mode energy into the receivingcircuit. As the rate of voltage change increases in that common-modeevent, the current injected into the receiver increases. This currentpotentially can damage the receiving circuit and can trigger a faultydetection. Such capacitively coupled circuitry thus couples signals thatshould be rejected. The patent also mentions, without elaboration, thata transformer with a short R/L time constant can provide an isolationbarrier, but such a differential approach is nonetheless undesirablebecause any mismatch in the non-magnetic (i.e., capacitive) coupling ofthe windings would cause a common-mode signal to appear as a differencesignal. Transformer cost and size may also be a negative factor, andtransformers having cores of magnetic materials such as ferrites becomeinefficient at high frequencies and are not useful for couplinghigh-speed digital signals.

Commonly-owned U.S. Pat. No. 5,952,849 shows another logic isolatorwhich avoids use of optical coupling. This logic isolator exhibits hightransient immunity for isolating digital logic signals.

A need exists, however, for a less expensive, higher performance digitalsignal isolator with good dynamic characteristics at high frequencies orspeeds.

Moreover, needs exist for logic isolators which provide improvedlow-cost bidirectional signal transmission capabilities and which can beconfigured for a variety of signal transmission configurations.

A need further exists for improved signaling schemes for use inisolators, to permit a logic isolator to be based on a singlemicro-transformer.

These needs are addressed with a logic signal isolator comprising, in afirst aspect, a transformer having a primary winding and a secondarywinding; a transmitter circuit which drives said primary winding inresponse to a received logic signal, such that in response to a firsttype of edge in the logic signal, a signal of a first predetermined typeis supplied to the primary winding and in response to a second type ofedge in the logic signal, a signal of a second predetermined type issupplied to said primary winding, the primary winding and thetransmitter being referenced to a first ground; and the secondarywinding being referenced to a second ground which is galvanically (i.e.,electrically) isolated from the first ground and said secondary windingsupplying to a receiver circuit signals received in correspondence tothe signals provided to the primary winding, the receiver reconstructingthe received logic signal from the received signals. The isolator'sreceiver may include circuitry for distinguishing between the receivedsignals corresponding to the transmitted signals of the first type andsecond type and using the distinguished received signals to reconstitutethe received logic signal. The signals of the first type may, forexample, comprise multiple pulses in a predetermined pattern and thesignals of the second type comprise one or more pulses in a differentpattern. The signals of the first type also may comprise pulses of afirst duration and the signals of the second type may comprise pulses ofa second, distinguishable duration. At least one of the signals of thefirst or the second type also may comprise at least one burst. If bothcomprise bursts, they may be at different frequencies or be fordifferent durations.

The transmitter circuit may be on a first substrate and the receiver maybe on a second substrate electrically isolated from the first substrate.

The primary winding and the secondary winding desirably may besubstantially planar windings arrange in a stacked arrangement with atleast one of the windings substantially in or on one of the substrates.The primary winding then may be a bottom winding (closer to thesubstrate) and the secondary winding may be a top winding (further fromthe substrate). When the primary winding is a bottom winding, theisolator may further include a compensation network connected to the topwinding for damping its response. Alternatively, the primary winding maybe a top winding and the secondary winding may be a bottom winding.

According to another aspect of the invention, a bi-directional isolatoris provided by including a second transmitter connected to drive saidsecondary winding in response to a second received logic signal, suchthat in response to a first type of edge in the second received logicsignal, a signal of a third predetermined type is supplied to thesecondary winding and in response to a second type of edge in the secondreceived logic signal, a signal of a fourth predetermined type issupplied to said secondary winding, the secondary winding and the secondtransmitter being referenced to the second ground; and the primarywinding being referenced to the first ground and said primary windingsupplying to a second receiver circuit signals received incorrespondence to the signals provided to the secondary winding, thesecond receiver reconstructing the second received logic signal. Theisolator's second receiver may include circuitry for distinguishingbetween the signals received from the primary winding and using thedistinguished received signals to reconstitute the second received logicsignal. The signals from the first transmitter and the secondtransmitter may be similar or different.

According to another aspect, a digital logic isolator comprises atransformer having a primary winding referenced to a first ground and asecondary winding referenced to a second ground isolated from the firstground, means for providing to the primary winding signals of a firsttype in response to transitions of a first type in an input logicsignal, means for providing to the primary winding signals of a secondtype different from the signals of the first type in response totransitions of a second type in the input logic signal, and means forreceiving from the secondary winding signals corresponding to thesignals of the first and second types and for reconstituting the inputlogic signal from them.

According to still anther aspect of the invention, a method of providingan isolated logic signal output in response to a logic signal inputcomprises providing a transformer having a primary winding referenced toa first ground and a secondary winding referenced to a second groundisolated from the first ground, providing to the primary winding signalsof a first type in response to transitions of a first type in the inputlogic signal, providing to the primary winding signals of a second typedifferent from the signals of the first type in response to transitionsof a second type in the input logic signal, and receiving from thesecondary winding signals corresponding to the signals of the first andsecond types and reconstituting the input logic signal from them.

The signals of a first type may comprise multiple pulses, the signals ofthe second type may comprise a single pulse and reconstituting the inputlogic signal may comprise distinguishing between the signalscorresponding to said multiple pulses and the signals corresponding tosaid single pulses so as to provide an output signal reconstituting thetransitions in the input logic signal. The signals of a first type orthe signals of a second type comprise a burst. If both the signals of afirst type and the signals of a second type comprise bursts, they may bedistinguishable from each other by frequency, duration or othercharacteristic. A signal of the first type alternatively may comprise apulse of a first duration and a signal of the second type may comprise apulse of a second duration different from the first duration anddistinguishable therefrom; and reconstituting the input logic signal maycomprise distinguishing between received signals corresponding to thepulses of a first duration and the pulses of a second duration so as toprovide an output signal reconstituting the transitions in the inputlogic signal.

According to another aspect of the invention, a logic isolator comprisesa micro-transformer comprising, on a substrate, vertically stacked, abottom winding and a top winding disposed over and insulated from thebottom winding, with a damping network connected across the top winding.A transmitter circuit receives a logic input signal and drives a signalto said bottom winding; and a receiving circuit is connected to receivefrom the top winding a signal corresponding to the signal driving thebottom winding and generates an output comprising a reconstituted logicinput signal.

According to still another aspect, a logic isolator comprises amicro-transformer having, on a substrate, vertically stacked, a bottomwinding and a top winding disposed over and insulated from the bottomwinding; a damping network connected across the top winding; atransmitter circuit receiving a logic input signal and providing atransformer driving signal; a receiving circuit connected to receivefrom the transformer a signal corresponding to the driving signal andgenerating an output comprising a reconstituted logic input signal; andmeans for programming the isolator by coupling the driving signal to aselected one of the windings and coupling the receiving circuit to theother one of the windings. In such an isolator, the means forprogramming may comprise a fusible connection(s) programmed by blowingopen a conductive path(s). The means for programming alternatively maycomprise bond wires provided between the transformer windings on the onehand and the transmitter and receiving circuits, on the other hand. As afurther alternative, the means for programming comprises programmablecircuitry configurable to connect the transmitter circuit to either thetop winding or the bottom winding and to connect the receiving circuitto the other winding. The programmable circuitry may includeprogrammable switching circuits and a memory containing programming tocontrol the switching circuits. The memory may be read-only memory.

According to yet another aspect, a logic isolator comprises amicro-transformer comprising, on a substrate, vertically stacked, abottom winding and a top winding disposed over and insulated from thebottom winding; a damping network connected across the top winding; afirst module coupled to the top winding capable of either receiving alogic input signal and providing a transformer driving signal orreceiving from the transformer a signal corresponding to the drivingsignal and generating an output comprising a reconstituted logic inputsignal; a second module coupled to the bottom winding capable of eitherreceiving a logic input signal and providing a transformer drivingsignal or receiving from the transformer a signal corresponding to thedriving signal and generating an output comprising a reconstituted logicinput signal; and means for programming the isolator such that the firstmodule operates in the transformer drive mode while the second circuitoperates in the receive mode or that the first module operates in thereceive mode while the second module operates in the transformer drivemode. Various alternatives may be used as the means for programming.Such means may comprise, for example, at least one fusible connectionprogrammed by blowing open at least one conductive path. As anotherexample, the means for programming may comprise one or more bond wiresprovided between the transformer windings on the one hand and the firstand second modules, on the other hand. The means for programming alsomay comprise programmable circuitry configurable to connect the firstmodule to either the top winding or the bottom winding and to connectthe second module to the other winding. Such programmable circuitry mayinclude programmable switching circuits and a memory containingprogramming to control the switching circuits. The memory may include aread-only memory.

According to still another aspect, a dual-channel, bi-directionalisolator comprises first and second micro-transformers arranged on afirst substrate, each transformer having a top winding and a bottomwinding. A first transmitter circuit is connected to drive the bottomwinding of the first transformer; a second transmitter circuit isconnected to drive the top winding of the second transformer. A firstreceiver circuit is connected to receive signals from the bottom windingof the second transformer. A second receiver circuit is connected toreceive signals from the top winding of the first transformer.Preferably, but not necessarily, the first transmitter circuit and firstreceiver circuit are on the first substrate, and the second transmittercircuit and second receiver circuit are on a second substrate which iselectrically isolated from the first substrate.

Yet another aspect of the invention is a single channel bi-directionalisolator comprising a micro-transformer arranged on a first substrate,the transformer having a top winding and a bottom winding; a firsttransmitter circuit connected to drive the bottom winding; a secondtransmitter circuit connected to drive the top winding; a first receivercircuit connected to receive signals from the top winding; a secondreceiver circuit connected to receive signals from the bottom winding;and the first and second transmitter circuits transmitting so as toavoid interfering with each other. Preferably, but not necessarily, thefirst transmitter circuit and the second receiver circuit are on thefirst substrate and the second transmitter circuit and the firstreceiver circuit are on a second substrate which is electricallyisolated from the first substrate.

According to still another aspect, there is provided a delay element foruse in pulse generating circuits for generating pulses usable, forexample, to drive a transformer as above-described. The delay element isuseful for generating a delay interval, and therefore a pulse duration,of a length that is substantially independent of the supplyvoltage—i.e., is insensitive to variations in supply voltage. The delayelement comprises first and second current sources which supply currentsI1 and I2, respectively, in parallel, and a switching element. The sumof currents I1 and I2 is directly proportional to the supply voltage,and a threshold of the switching element is a predetermined portion ofthe supply voltage. The delay element may further include a capacitor ofcapacitance C, connected to a node in common with the input of theswitching element and the current sources, chargeable by the currentsources. Preferably, the first current source comprises a singletransistor and a resistor, the resistor, of resistance value R, havingone end connected to the supply voltage and the other end connected tosaid transistor. Current I1=(VDD−VT)/R, where the transistor is a MOStransistor, VT is the threshold voltage of the MOS transistor, VDD isthe supply voltage, I2=VT/R, I1+I2=VDD/R. The delay interval is thenapproximately 0.5 RC if the switching threshold of the switching elementis set to be VDD/2, and is relatively insensitive to changes in VDD.Such a delay element may be used in conventional pulse generatingcircuits that rely upon use of a delay element.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages will occur to those skilled inthe art from the following description of a preferred embodiment and theaccompanying drawings, in which:

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 is a simplified schematic circuit diagram of a transformer-basedisolator according to the prior art;

FIG. 2 is a simplified schematic circuit diagram of a glitch filter foruse in an isolator according to the invention;

FIG. 3 is a set of waveforms for the circuit of FIG. 2;

FIG. 4 is a simplified schematic circuit diagram of a first embodimentof a transformer-based isolator embodying aspects taught herein;

FIG. 5 is a set of waveforms for the circuit of FIG. 4;

FIG. 6 is a simplified schematic circuit diagram of a second embodimentof a transformer-based isolator embodying aspects taught herein.

FIG. 7 is a waveform depicting two distinguishable pulses of differentpulse width, such as may be used in an alternative embodiment of anisolator as taught herein;

FIG. 8 is a diagrammatic illustration of an isolator according to someaspects of the invention, wherein the primary winding is a top winding;

FIG. 9 is an illustration of input and output waveforms when driving amicro-transformer of the type preferably employed in implementingembodiments of the isolators taught herein, particularly illustratingthe difference between driving top and bottom transformer windings;

FIG. 10 is a diagrammatic illustration of an isolator according to someaspects of the invention, wherein the primary winding is a bottomwinding;

FIG. 11 is a diagrammatic illustration of a bi-directional dual channelisolator such as may be implemented using the teachings herein;

FIG. 12 is a simplified schematic circuit diagram for asupply-independent delay element for use in a pulse generator(transmitter) in isolators such as are taught herein; and

FIGS. 13 and 14 are simplified schematic circuit diagrams for currentsources for use in the delay element of FIG. 12.

DISCLOSURE OF THE PREFERRED EMBODIMENT

Aside from the preferred embodiment or embodiments disclosed below, thisinvention is capable of other embodiments and of being practiced orbeing carried out in various ways. Thus, it is to be understood that theinvention is not limited in its application to the details ofconstruction and the arrangements of components set forth in thefollowing description or illustrated in the drawings. If only oneembodiment is described herein, the claims hereof are not to be limitedto that embodiment. Moreover, the claims hereof are not to be readrestrictively unless there is clear and convincing evidence manifestinga certain exclusion, restriction, or disclaimer.

Although specific features of the invention are shown in some drawingsand not in others, this is for convenience only as each feature may becombined with any or all of the other features in accordance with theinvention. The words “including”, “comprising”, “having”, and “with” asused herein are to be interpreted broadly and comprehensively and arenot limited to any physical interconnection. Moreover, any embodimentsdisclosed in the subject application are not to be taken as the onlypossible embodiments. Other embodiments will occur to those skilled inthe art and are within the following claims.

In addition, any amendment presented during the prosecution of thepatent application for this patent is not a disclaimer of any claimelement presented in the application as filed: those skilled in the artcannot reasonably be expected to draft a claim that would literallyencompass all possible equivalents, many equivalents will beunforeseeable at the time of the amendment and are beyond a fairinterpretation of what is to be surrendered (if anything), the rationaleunderlying the amendment may bear no more than a tangential relation tomany equivalents, and/or there are many other reasons the applicant cannot be expected to describe certain insubstantial substitutes for anyclaim element amended.

This invention is not limited in its application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other embodiments and of being practiced or of being carriedout in various ways. Also, the phraseology and terminology used hereinis for the purpose of description and should not be regarded aslimiting. The use of “including,” “comprising,” or “having,”“containing”, “involving”, and variations thereof herein, is meant toencompass the items listed thereafter and equivalents thereof as well asadditional items.

Micro-transformer based digital isolators have been developed in recentyears by applicants and their colleagues. This genus of digitalisolators shows dramatic improvements over traditional opto-isolators interms of speed, power, edge symmetry and cost. The transmission methodsemployed in these micro-transformer based digital isolators fall intotwo main categories. The first category is based on edge detection; thesecond, on level detection. Those designs that are based on edgedetection have the advantage of lower power, lower pulse widthdistortion and higher common mode transient immunity over those based onlevel detection. Typically, implementations based on edge detectionrequire two transmitters, two receivers and two transformers to make asingle channel isolator. A need exists for a less expensive design.

As shown herein, a digital isolator may be formed, which is based on amicro-transformer-created isolation barrier, using only a singletransmitter, a single receiver and a single transmitter. This approachdramatically reduces the die cost while still preserving the merits ofedge detection. Further, in a vertically stacked arrangement ofmicro-transformer windings, the present invention enables bi-directionalsignal transfer. That is, the signal can be transferred from the topcoil to the bottom coil or from the bottom coil to the top coil. Thiscapability can be leveraged to make bi-directional, multi-channel signalisolators or to program the channel direction of a single data channel.This invention preserves the main advantages of high speed, low power,high common-mode immunity and adds to them a reduction in size andenhanced ease of integration.

By the term “micro-transformer,” there is meant transformer formed in,partially in, or on, a semiconductor substrate of flat, parallelconductive windings and having no magnetic core. These transformers arealso referred to as “air-core” transformers though there actually willbe more than air between the windings, typically one or more layers ofdielectric materials.

A block diagram of a typical prior art example of a transformer-basedisolator for digital signals is shown in FIG. 1. There, the isolator 100comprises (after an optional, though preferable, glitch filter 101), apair of edge detectors 102 and 104, edge detector 104 being driven bythe logical complement to the signal driving edge detector 102, by meansof inverter 106. The output of edge detector 102 provides a pulse, theSET_HI signal to the primary winding of transformer 108 responsive todetection of a low-to-high transition (i.e., a leading edge) in theoutput of the glitch filter 101. The secondary winding of transformer108 is connected to the set input of a flip-flop 110. The output of edgedetector 104 is a pulse named the SET_LO signal which is supplied to theprimary winding 120A of transformer 120 responsive to detection of ahigh-to-low transition (i.e., a falling edge) in the output of theglitch filter 101. The secondary winding of transformer 120 is suppliedto the reset input of flip-flop 110. Typically, though not shown for thesake of avoiding obfuscation, the connection from each transformersecondary winding to the flip-flop 110 is not direct, but is madethrough a Schmitt trigger or other waveform-shaping circuitry intendedto provide clean, fast transitions to the flip-flop.

Note that the primary windings 108A and 120A of transformers 108 and120, respectively, as well as the glitch filter and the two edgedetectors, are connected (referenced) to a first electrical ground,GNDA, while the secondary windings 108A and 120B, together with theflip-flop 110, are referenced to a second electrical ground, GNDB, whichis isolated from the first ground.

The outputs of the edge detectors 102 and 104 comprise encoded leadingedge and falling edge indicators. These indicators may be in the form ofpulses, short bursts, or any periodic signal. So, the edge detectors maybe monostable multivibrators, differentiators or other suitablecircuits. In the illustration, a single pulse of about lns duration isshown as an edge indicator signal.

Glitch filter 101 may be of any suitable design. A brick wall filter istypical. Typically, it should have a bandwidth larger than that to whichthe edge detectors respond. FIG. 2 provides a schematic circuit diagramfor a usable filter, corresponding timing diagrams showing the waveformsat the input, nodes A-D and the filter output being provided in FIG. 3.The filtered pulse width is given approximately by CVthreshold/i, whereVthreshold is the threshold voltage of the Schmitt triggers 22 and 24and i is the current from current sources 26, 28.

Turning now to FIG. 4, a basic embodiment for practicing the presentinvention is shown. FIG. 4 illustrates a logic signal isolator 200 inwhich encoded leading and falling edge indicators from a pair of edgedetectors 202 and 204 (corresponding to edge detectors 102 and 104 ofFIG. 1) are sent to a single transformer 210. Unlike the above-discusseddesign, however, the leading edge and falling edge indicators areencoded as different, distinguishable signals. That is, the SET_HIsignal output from leading edge detector 202 is distinct from the SET_LOsignal output from falling edge detector 204. The receiving sidecircuitry connected to the secondary winding 210B of transformer 210(again, typically via a Schmitt trigger or other suitable wave-shapingcircuit, not show) then reconstructs the logic edges based ondistinguishing between those two signals.

An example is illustrated wherein edge detector 202 produces twoconsecutive short pulses 232 and 234 as a leading edge indicator andedge detector 204 produces only a single pulse 236 as a falling edgeindicator. The pulses 232 and 234 preferably have a known, fixed spacingbetween them. If transformer 210 is a high bandwidth micro-transformer,the pulse widths may be as narrow as 1 ns or even less. The outputs ofedge detectors 202 and 204 are combined by and OR gate 240, to drive theprimary winding 210A of the transformer. The pulses cannot be so shortor weak in amplitude that the OR gate will not respond properly.

Of course, the concept is to use two different, distinguishable signals.They need not be a single pulse and a double pulse. For example, anarrow pulse (e.g., 1 ns) could be used as one edge indicator and awider pulse (e.g., 2 ns) could be used as the other edge indicator. Itis only necessary that the receiver be able to distinguish the twosignals. The concept lends itself to the use of other distinguishablesignals but at the same time, one would not wish to use an unnecessarilycomplicated arrangement or one which would add any significant delay insignal processing. For signals other than those illustrated, it might benecessary to replace OR gate 240 with other elements that wouldeffectively combine the outputs of the edge detectors into a singlesignal for driving the transformer.

The two pulses in the SET_HI signal have a known, fixed spacing betweenthem. The total duration of the two pulses and the intervening gapbetween them in the SET-HI signal, if sufficiently short with respect tothe shortest interval between two leading edges in the input signal,will permit resolution between the SET-HI and SET_LO pulses.

A receiver circuit 250, connected to secondary winding 210B, recoversthe output of transformer 210, distinguishes between the SET_HI andSET_LO pulses, and reconstructs the input logic signal as a data outsignal. More specifically, the received pulses at node 252 clock aD-type flip-flop 254 and also act as the input to a non-retriggerableedge-triggered mono-stable multivibrator 256. The multivibrator 256 putsout a pulse on line 258 that is of duration at least as long as thecombination of pulse 232 and the interval between pulse 232 and pulse234 in the SET_HI signal. If the two pulses 232 and 234 are eachapproximately 1 ns in duration and the interval between them is of likeduration, then the pulse on line 258 should be at least about 2 ns long;3 ns is used in this example to allow some “hold” time to facilitateclocking of flip-flop 254. Line 258 connects to the D input of flip-flop254, to the reset input of that flip-flop and to the input of inverter262. The output of inverter 262 is connected to the input of an edgedetector 264 and the QB output (the complementary output) of flip-flop254 is connected to the input of another edge detector 266. The outputof edge detector 264 is connected to one input of each of AND gates 272and 274. The output of edge detector 266 is connected to the secondinput of AND gate 272 and through inverter 276 to the second input ofAND gate 274. In turn, the output of AND gate 272 is connected to theset input of set/reset flip-flop 278 and the output of AND gate 274 isconnected to the reset input of flip-flop 278. The DATA OUT signal,corresponding to an isolated and slightly delayed version of the DATA INsignal received by the glitch filter, appears at the Q output offlip-flop 278.

The operation of this circuit will now be explained with reference tothe waveforms of FIG. 5. Assume that the DATA IN input has the waveform302. At node 252, the COIL signal is received. Pulses 232 and 234 havebeen generated by edge detector (i.e., transmitter) 202 in response tothe leading, positive-going edge of the input signal and pulse 236 hasbeen generated by edge detector 204 in response to the negative-going,trailing edge of the input signal. The edge-triggered mono-stable (ETMS)multivibrator 256 generates an output waveform on line 258 as shown atETMS. In the ETMS signal, the leading edge of pulse 232 causes the pulse304 to be generated. The multivibrator 256 does not do anything inresponse to the falling edge of pulse 232 or to either edge of thesecond pulse 234. Only after pulse 304 has been output is themultivibrator 256 able to respond to a new input, which it does when itreceives the leading edge of pulse 236. Detection of the leading edge ofpulse 236 causes the outputting of pulse 306.

The second of the two initial pulses, pulse 234, is detected and theoutput signal is formed as follows. When the first pulse 232 clocks theflip-flop 254, the D input of the flip-flop still sees a low output fromthe edge-triggered mono-stable multivibrator on line 258. That means theQB output of the flip-flop 254 is set to a high value and the Q outputis set to a low value. When the second pulse 234 is received and clocksflip-flop 254, the output of the edge-triggered mono-stable is now highand the QB output of flip-flop 254 transitions to a low value, meaningthat the Q output of flip-flop 254 goes high as at the leading edge ofthe pulse 308 in the “2 Pulse Detect” signal on FIG. 5. This H-L edge issensed by edge detector 266, which produces a pulse 310 to the second(bottom) input of AND gate 272. The output of the edge-triggeredmono-stable is also supplied to the input of inverter 262. So, after thepropagation delay through inverter 262, edge detector 264 sees ahigh-to-low transition (edge) at the output of inverter 262 andresponsively generates a positive-going pulse 312 to the first (top)input of AND gate 272 and to a first (top) input of AND gate 274.Inverter 262 is designed to have a propagation delay that issubstantially equal to that from the D input to the QB output offlip-flop 254. Hence, edge detectors 264 and 266 produce substantiallyconcurrent output pulses 310 and 312 to AND gate 272. As a result, theoutput 314 of AND gate 272 goes from low to high at the same time andsets the set (S) input of the SR flip-flop 278; and the Q outputthereof, being the DATA OUT signal, goes high. As the second (bottom)input of AND gate 274 is responsive to the output of edge detector 266through inverter 276, the first and second pulses have no impact on theoutput of AND gate 274 and do not affect the output of flip-flop 278.However, when third pulse 236 triggers edge-triggered mono-stable 256,to produce its second output pulse 306, this results as stated above, inthe generation of a pulse at the output of edge detector 264 upon thefalling edge of the mono-stable output pulse. The second input of ANDgate 274 from inverter 276 will be high at this time because the onlytime it is low is when the output of edge detector 266 generates thesecond pulse detection signal 308. Therefore, the reset (R) input offlip-flop 278 sees the output pulse 316 from AND gate 274 upon thefalling edge of the output pulse from edge detector 264 (pluspropagation delay), and flip-flop 278 is reset and the DATA OUT signalgoes low.

An alternative embodiment 200′ for the pulse receiver circuitry is shownin FIG. 6. Edge detectors 264, 266 and gates 272, 274 and 276 have beeneliminated and the output flip-flop 278′ is changed from a set-resetflip-flop to a D-type flip-flop. Again, the first pulse 232 clocksflip-flop 254 before the edge-triggered mono-stable 256 has generated anoutput pulse on the D input of flip-flop 278′. Therefore, the Q outputof flip-flop 254 assumes a low state. When the second pulse 234 clocksflip-flop 254, the D input thereof now sees the output pulse 304 fromthe edge-triggered mono-stable and the Q output of flip-flop 254transitions to a high value. The falling edge of the mono-stable pulse304 is coupled to the clock input of flip-flop 278′ through inverter262, and clocks flip-flop 278′; as a result, the high value from the Qoutput of flip-flop 254 supplies a high value on the Q output offlip-flop 278′, and the DATA OUT signal. The falling edge of themono-stable output pulses also reset flip-flop 254 via its RB (i.e., Rcomplement) input. The output flip-flop 278′ is next clocked, again, bythe falling edge of the output pulse from the mono-stable, being edge306B, of pulse 306 generated in response to the third pulse, 236, on thetransformer. At the time of edge 306B, flip-flop 254 has been reset tohave a low output and the output of flip-flop 278′ accordingly goes low.

To assure proper operating states, a reset signal termed PWReset_B issupplied to the reset (complement) input of flip-flop 278′ and causesflip-flop 278′ to be reset whenever device power is reset.

An alternative signaling arrangement, mentioned above, is shown in FIG.7. There, instead of using two pulses to indicate a rising edge and onepulse to indicate a falling edge in the input signal, a double-widthpulse 350 is used to indicate a rising edge and a single-width pulse 360is used to indicate a falling edge. Those skilled in the art ofelectrical engineering will readily be able to device logic circuitry todiscriminate between a pulse of single width duration Δ and a pulse ofdouble width duration 2Δ.

An exemplary physical implementation for an isolator according to thepresent invention, capable of being packaged in an integrated circuitform, is shown in FIG. 8. There, a transmitter (or driver) circuit 802is formed on a first substrate 804. A transformer comprising a firstwinding 806A and a second winding 806B is formed on a second substrate808, along with a receiver circuit 810. Wire leads 812A, 812B from bondpads 814A and 814B on substrate 804 connect the driver output to theprimary winding 806A of the transformer. As shown there, obviously, theprimary (driving) coil winding is the top coil 806A and the secondary(receiving) coil winding is the bottom coil 806B. It is important tonote that the two coils, even if made in the same dimensions andgeometry, will exhibit different quality factor Q, because the bottomcoil has higher capacitance to substrate 808.

Referring to FIG. 9A, when an idealized square voltage pulse 902 drivesthe top winding 904 of such a transformer, the voltage received at thebottom winding 906 will exhibit a waveform something like 908. Bycontrast, if the bottom coil 906 is driven by the idealized pulse 912 asshown in FIG. 9B, then the received voltage waveform at the top coil 904will typically be as shown at 914, exhibiting ringing due to the factthat the top coil has a higher Q than the bottom coil. This undesiredringing makes it difficult to use the transformer for bi-directionalsignal transfer. However, bi-directional signal transfer is desired inat least some embodiments of the present invention and it is possible toestablish a greater degree of symmetry as shown in FIG. 9C, by adding adamping network 916 on the receiving top coil. The damping networkconsists of a resistor 922 and a capacitor 924 in series. Damping with aresistor only would dramatically reduce the received signal andtypically would not be acceptable. The capacitor 924 has very lowimpedance at high frequencies and blocks DC current through the dampingresistor, providing the desired response characteristics. Note that withthe damping network, the received waveform 926 from the top coil issubstantially the same as the received waveform from the bottom coil asshown at 908 in FIG. 9A.

Through use of such a damping network, the edge-detection based isolatorabove described can be implemented as pictured in FIG. 10. There, it canbe seen that the primary coil is the bottom coil 906 instead of the topcoil 904.

A bi-directional dual isolator arrangement as shown in FIG. 11 is thusenabled. This bi-directional isolator has a pair of stacked transformersarranged side by side. A first transformer is formed by the windings1102A and 1102B, while a second transformer is formed by the windings1104A and 1104B. Only one substrate 1106 carries the transformerstructure because the primary winding now can be the top coil or bottomcoil of a transformer. Absent this possibility, each of the twosubstrates 1106 and 1108 would need to have an isolated transformerstructure thereon and the product would be considerably more expensiveto make.

Alternatively, it is possible to make a single channel bi-directionalisolator having only one vertically stacked transformer, with onetransmitter driving the transformer while the other transmitter isidled. Synchronization of the transmissions in two directions can beprogrammed externally or through proper command encoding/decoding.

For some applications, where bandwidth and data rate are not paramountconsiderations, instead of using pulses to drive the transformerprimaries, other signals such as analog bursts at predeterminedfrequencies and of predetermined durations may be employed. In suchsituations, signals can be transmitted bi-directionally concurrentlythrough a single transformer.

Advantageously, single- and multiple-channel isolators can bemanufactured so that the selection of configuration (i.e., whichchannels transmit in which directions) can be made in final assembly andtest steps of production. That lowers product cost by allowing oneproduct core to be made and sold for multiple configurations. Thedesigns shown above lend themselves to this approach in one of two ways.In a first approach, a logic isolator comprises a micro-transformerhaving, on a substrate, vertically stacked, a bottom winding and a topwinding disposed over and insulated from the bottom winding; a dampingnetwork connected across the top winding; a transmitter circuitreceiving a logic input signal and providing a transformer drivingsignal; a receiving circuit connected to receive from the transformer asignal corresponding to the driving signal and generating an outputcomprising a reconstituted logic input signal; and means for programmingthe isolator by coupling the driving signal to a selected one of thewindings and coupling the receiving circuit to the other one of thewindings. In general, the programming would have to be effected beforefinal testing of the isolator in order to maintain isolation betweeninput and output. In such an isolator, the means for programming may,for example, comprise a fusible connection(s) programmed by blowing opena conductive path(s). The means for programming alternatively maycomprise bond wires provided between the transformer windings on the onehand and the transmitter and receiving circuits, on the other hand. Ineither instance, the isolator cannot be tested untilisolation-destroying paths are blown open or isolation-destroying bondwires are removed (if there had been any); of course, bond wires couldselectively be installed as the last step in manufacture, beforetesting. As a further alternative, the means for programming maycomprise programmable circuitry configurable to connect the transmittercircuit to either the top winding or the bottom winding and to connectthe receiving circuit to the other winding. Again, however, only one setof valid connections can be established if input-output isolation is tobe maintained. The programmable circuitry may include programmableswitching circuits and a memory containing programming to control theswitching circuits. The memory may be read-only memory. A secondapproach may be based on providing modules, each having both atransformer-driving circuit (i.e., transmitter) and a receiver circuit,such that the module is configured to operate only as a driving circuitor only as a receiving circuit, configuring done at final assembly or bythe user. In this approach, a logic isolator comprises amicro-transformer having, on a substrate, vertically stacked, a bottomwinding and a top winding disposed over and insulated from the bottomwinding; a damping network connected across the top winding; a firstmodule coupled to the top winding capable of either receiving a logicinput signal and providing a transformer driving signal or receivingfrom the transformer a signal corresponding to the driving signal andgenerating an output comprising a reconstituted logic input signal; asecond module coupled to the bottom winding capable of either receivinga logic input signal and providing a transformer driving signal orreceiving from the transformer a signal corresponding to the drivingsignal and generating an output comprising a reconstituted logic inputsignal; and means for programming the isolator such that the firstmodule operates in the transformer drive mode while the second circuitoperates in the receive mode or that the first module operates in thereceive mode while the second module operates in the transformer drivemode. Various alternatives may be used as the means for programming.Such means may comprise, for example, at least one fusible connectionprogrammed by blowing open at least one conductive path. As anotherexample, the means for programming may comprise one or more bond wiresprovided between the transformer windings on the one hand and the firstand second modules, on the other hand. The means for programming alsomay comprise programmable circuitry configurable to connect the firstmodule to either the top winding or the bottom winding and to connectthe second module to the other winding. Such programmable circuitry mayinclude programmable switching circuits and a memory containingprogramming to control the switching circuits. The memory may include aread-only memory.

Utilizing these approaches, a dual-channel, bi-directional isolatorcomprises first and second micro-transformers arranged on a firstsubstrate, each transformer having a top winding and a bottom winding. Afirst transmitter circuit is connected to drive the bottom winding ofthe first transformer; a second transmitter circuit is connected todrive the top winding of the second transformer. A first receivercircuit is connected to receive signals from the bottom winding of thesecond transformer. A second receiver circuit is connected to receivesignals from the top winding of the first transformer. Preferably, butnot necessarily, the first transmitter circuit and first receivercircuit are on the first substrate, and the second transmitter circuitand second receiver circuit are on a second substrate which iselectrically isolated from the first substrate.

Similarly, a single channel bi-directional isolator comprising amicro-transformer arranged on a first substrate, the transformer havinga top winding and a bottom winding; a first transmitter circuitconnected to drive the bottom winding; a second transmitter circuitconnected to drive the top winding; a first receiver circuit connectedto receive signals from the top winding; a second receiver circuitconnected to receive signals from the bottom winding; and the first andsecond transmitter circuits transmitting so as to avoid interfering witheach other. Preferably, but not necessarily, the first transmittercircuit and the second receiver circuit are on the first substrate andthe second transmitter circuit and the first receiver circuit are on asecond substrate which is electrically isolated from the firstsubstrate.

Such isolators typically have to work with a wide range of supplyvoltage. If such a characteristic is desired, and the driving signalsare to be pulses, then it is also necessary that the transmitters (i.e.,edge detectors or pulse generators of whatever nature) be able togenerate pulses of precise, voltage-independent pulse width. Methods togenerate such voltage-independent pulse widths will now be discussed. Adelay element typically is required in such pulse generators. And asillustrated schematically in FIG. 12, a delay element 1200 comprises twocurrent sources 1202 and 1204 which supply currents I1 and I2,respectively, one capacitor, C, and one switching element such as aninverter or a Schmitt trigger 1206. The sum of currents I1 and I2 issuch that it is directly proportional to the supply voltage, and theswitching threshold is half or some other constant portion of the supplyvoltage. A simplified example of a current source 1202 for generatingcurrent I1 is shown in FIG. 13. There, current source 1202 comprises asingle transistor 1212 and a resistor R. The resistor R has one endconnected to the supply voltage VDD and the other end connected to thedrain of transistor 1212.

Current I1=(VDD−VT)/R, where VT is the threshold voltage of the MOStransistor 1212. Current I2=VT/R, I1+I2=VDD/R. Consequently, the delaytime will be 0.5 RC if the switching threshold is set to be VDD/2.Current source 1204 for generating current I2 can be easily implemented.For example, a simplified schematic circuit diagram for such a currentsource is shown in FIG. 14. Transistor 1214 imposes the voltage VTacross resistor R, inducing current I2 and that current is mirrored inthe drain path of transistor 1214.

The delay element 1200 can be easily used to generate pulses whose widthis independent of the supply voltage.

Of course, the illustrated design need not be used to form the requiredcurrent source. Any design can be used that will produce asupply-independent pulse width. In the examples shown, this is achievedby adding a first current that is proportional to the supply voltage toa second current that is inversely proportional to the supply voltage.Other approaches are not ruled out.

Having discussed the principles involved and having illustrated multipleembodiments, it will be further apparent that various alterationsthereto and additional embodiments will occur to those skilled in theart. Any such alterations, amendments, improvements and additionalembodiments are intended to be within the spirit and scope of theinvention, which is not limited by the foregoing examples but only asrequired by the appended claims and equivalence thereto.

1. A logic signal isolator comprising: a transformer having a primarywinding and a secondary winding; a transmitter circuit which drives saidprimary winding in response to a received logic signal, such that inresponse to a first type of edge in the logic signal, a signal of afirst predetermined type is supplied to the primary winding and inresponse to a second type of edge in the logic signal, a signal of asecond predetermined type is supplied to said primary winding, theprimary winding and the transmitter being referenced to a first ground;and the secondary winding being referenced to a second ground which isgalvanically isolated from the first ground and said secondary windingsupplying to a receiver circuit signals received in correspondence tothe signals provided to the primary winding, the receiver reconstructingthe received logic signal from the received signals.
 2. The isolator ofclaim 1, wherein the receiver includes circuitry for distinguishingbetween the received signals corresponding to the transmitted signals ofthe first type and second type and using the distinguished receivedsignals to reconstitute the received logic signal.
 3. The isolator ofclaim 1 or claim 2, wherein the signals of the first type comprisemultiple pulses in a predetermined pattern and the signals of the secondtype comprise one or more pulses in a different pattern.
 4. The isolatorof claim 1 or claim 2, wherein the signals of the first type comprisepulses of a first duration and the signals of the second type comprisepulses of a second, distinguishable duration.
 5. The isolator of claim 1or claim 2, wherein at least one of the signals of the first or thesecond type comprise at least one burst.
 6. The isolator of claim 1 orclaim 2, wherein the transmitter circuit is on a first substrate and thereceiver is on a second substrate electrically isolated from the firstsubstrate.
 7. The isolator of claim 6, wherein the primary winding andthe secondary winding are substantially planar windings arrange in astacked arrangement with at least one of the windings substantially inor on one of the substrates.
 8. The isolator of claim 7, wherein theprimary winding is a bottom winding and the secondary winding is a topwinding.
 9. The isolator of claim 8 further including a compensationnetwork connected to the top winding.
 10. The isolator of claim 7wherein the primary winding is a top winding and the secondary windingis a bottom winding.
 11. A digital logic isolator, comprising: a. atransformer having a primary winding referenced to a first ground and asecondary winding referenced to a second ground isolated from the firstground; b. means for providing to the primary winding signals of a firsttype in response to transitions of a first type in an input logicsignal; c. means for providing to the primary winding signals of asecond type different from the signals of the first type in response totransitions of a second type in the input logic signal; and d. means forreceiving from the secondary winding signals corresponding to thesignals of the first and second types and for reconstituting the inputlogic signal from them.